Surface properties of gonococci (Gc) are markedly influenced by their possessing pili whose structural/functional characteristics may vary widely. Gc exhibit high-frequency changes both in their piliation status (pilus + or pilus-) and in the pilin subunits which constitute their mature pili. Both "on-off" transitions and subunit size variation of Gc pili are being studied to discover their control mechanisms. These transitions were examined for colonial morphotype, piliation (by SEM), pilin production and subunit size (by immunoblotting), pilin-mRNA (by Northern blotting), pilin gene-containing DNA rearrangements (by Southern blotting), cloning and sequencing of Gc pilin gene DNA, and mRNA sequencing. We have identified and differentiated several pilus- phenotypes--both reverting (to pilus+) and nonreverting--and have identified the changes in pilin gene sequence that accompany their lack of ability to produce pilin subunit that can be incorporated into bonafide pili of Gc.